The overhead lines and cables widely used for the transmission of power and energy, of data and of lighting conducting cables are susceptible to ruptures due to fatigue caused by aeolian vibrations, that is, the vibration caused by the action of the wind acting upon cables.
When the wind travels on and along the overhead cables, an alternate unleashing of vortices of Von Kámán takes place which, consequently, produces alternate forces and pressures over the cables and cause mechanical vibrations.
These vibrations, which act permanently upon the cables may cause micro-fractures in the cables near the points where this movement is limited, such as, for example, in the proximity of the network of isolators suspended in the metallic towers, suspension staples, aerial signalling spheres, pre-fabricated connections, the actual places where the mechanical impedance is actually high.
With time these micro fractures grow in size and agglutinate until they reach a catastrophic growth velocity and cause a sudden rupture of the cable with all its economic and social consequences.
In view of the above, it is of utmost importance to reduce the vibrations on the cables by providing an efficient, low cost and durable device for neutralizing these vibrations which can be easily attached to the cables of overhead transmission lines.
The device for dynamically neutralizing vibrations according to the present invention complies with the all the above mentioned provisions and due to its generous viscous-elastic buffering, provided by the use of an elastomeric material particularly designed for this specific use, it easily distinguishes from the classic Stockbridge solution which presents a very low intrinsic buffering and, therefore, provides an excellent performance regarding the reduction of vibrations in a wide range of frequencies, covering all ranges of wind velocity.
The forces or excitations produced on the cables by the unleashing of Von Kámán's vortices are characterized by being random in time, depending on wind velocity and direction and its distribution along the cables. In scientific terms, it is said that these forces constitute a stochastic process of narrow range. While these forces have a rather narrow spectral content, that is, for each wind velocity its dependency of the wind velocity in frequency and amplitude makes the resonating vibrations along a wide range of frequencies, typically 5 to 150 Hz, they are able to be found on the cables in different moments in time.
Typically, two different approaches have been used to reduce the Aeolian vibrations in overhead cables and lines. The first one consists in the use of lines with large catenaries and is based on the fact that the intrinsic buffering is reduced by the increase of the mechanical traction acting on the cable due to its stretching because of its sheer weight. As such, stretching tractions are used to tension the cables, reaching a maximum of 20% of its rupture tension.
The second approach is the use of additional devices, known as dynamic vibration neutralizers, also known as buffers or even dynamic vibration absorbers, which are fixed to the vibrating cable with the purpose of reducing the vibrations caused by the vortices. These devices act by applying reactive forces and/or causing the dissipation of the vibratory energy.
A number of devices presently being used derive from the so-called Stockbridge buffer or neutralizer, which original design was patented by George H. Stockbridge in 1928 under U.S. Pat. No. 1,675,391, which consists of a symmetric or asymmetric neutralizers having a mechanical system with one, two or four degrees of freedom of movement. The device comprises a central mounting staple, two masses, equal or not, and two rods, generally metallic, equal or not, which assemble the above-mentioned elements. In this category of neutralizers one can list different types of devices which can be readily described in the following patents:
Author/InventorPatentCountryGeorge H. Stockbridge1,675,391USPhilip Wellesley Dulhunty3,711,624USPhilip Wellesley Dulhunty4,159,393USChristopher Francis Beard2,068,081 AUKAlberto Perinetti4,620,060USGiorgio Diana0,345,567 A1EPOPhilip Wellesley DulhuntyWO 02/07278 A1PCT
It is also possible for one to come across devices which, in a symmetrical form or non symmetrical, and of essential or accessorial form, encompass elastomeric materials in its composition. Such devices are known from following patents:
Author/InventorPatentCountryRaymond R. Bouche4,011,397USCharles Mathieu4,167,646USRaymond R. Bouche4,259,541USCharles B. Rawlins4,523,053USJosé J. de Espíndola et alPI 9905252-0BR
Another category of the devices in use is the spiral vibration buffers, nothing more than spiral objects with the cable running through the middle of it, longitudinally. These devices are pre-fabricated and act as impact buffers or shock absorbers, striking the vibrating cable and are covered by the following patent:
Author/InventorPatentCountryThomas Sherman4,620,059US
One can still find devices in which the vibrating energy dissipation takes place due to the friction of spring elements helicoidally shaped with the following patent:
Author/InventorPatentCountryPaul D. Turtle4,140,868US
And finally, there are devices produced for the reduction of the vibration in two or more cables, simultaneously, according to the following patents:
Author/InventorPatentCountryLeopold Leblanc3,906,143USOlaf Nigol, Herbert J.Houston4,362,900USOlaf Nigol, Herbert J.Houston4,384,166US